HISTOLOGICAL CHANGES ACCOMPANYING ADVENTITOUS SHOOTS FROMATION OF FRAGRAIA X ANANASSA IN VITRO LEAVES

A tissue culture experiment was conducted to attain the adventitious buds initiation and growth from Fragraia x ananassa cv. Festival in vitro leaves. The leaves were excited from shoot propagated in vitro and cultured on Murashige and Skoog (MS) medium supplemented with 4.50, 6.75, and 9.00 mmol l-1 thidiazuron (TDZ) for six weeks. Then explants were transferred to MS medium (salts and vitamins) free plant growth regulators for 8 weeks. This study included the histological and morphological responses of strawberry leaves with TDZ treatments in relation to some biochemical components. All TDZ treatments resulted in adventitious shoots development on leaf explants without callus formation. Growth in 6.75 mmol l-1 TDZ led to an increment in the most morphological parameters compared to the rest treatments. The results showed that 6.75 mmol l-1 of TDZ treatment significantly increases the number of shootlets per explant (5.0), total shoot (51.3), the number of leaves/explant (12) as compared with other treatments while the rest of parameters (the survival percentage, shoot length (cm), number of leaves/explant, root length (cm), and number of roots/explant) did not show any significant differences. Accumulation of chlorophylls, total sugars, and total soluble phenols was enhanced by 6.75 mmol l-1 of TDZ. The histological observations showed that adventitious shoots initiated from some parenchyma of ground tissue in the midvein zone of the explant, whereas the parenchyma mesophyll and the epidermal cells did not involve in this process. The ontogenesis of adventitious shoots was described by successive stages with the following distinguishable anatomical structures: meristemoids, bud primordium, shoot apex with leaf primordial, branching of adventitious shoots. After four weeks of culture, cell dedifferentiation was recorded in the midvein parenchyma and forming small groups of divided cells called meistemoid centres. These centres forming new meristematic masses embedded in the ground tissue of the main vein. After six weeks, further development of these masses resulted in the formation of bud primordial with normal organized shoot apical meristems and leaf primordial and arising from the explant as small protrusions. After two weeks from subculture on MS free TDZ, the adventitious shoots continue to elongate with forming new lateral branches and clearly observed on the adaxial side of the explant. This procedure provides a simple and rapid approach to regenerate strawberry plants via direct organogenesis.